Williamson Amplifier

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TECH LINE (702) 565-3993 M-Th 8 am to 4 pm (PST)

The Williamson

Amplifier

A Collection of Articles, reprinted from" Wireless World," on

"Design for a High-quality Amplifier"

By

D. T. N. WILLIAMSON (formerly of the M.O. Valve Company, now with Ferranti Research Laboratories)

Published for

Winless I'Qrld

•

LONDON: ILIFFE & SONS, LTD.

THIRD EDITION

THIRD PRINTING

1994

Copyright © 1990 by Audio Amateur Publications, Inc.

Distribution Agents: Old Colony Sound Lab

Post Office Box 243

Peterborough, New Hampshire 03458-0243

USA

Library of Congress Card Catalog Number: 90-083998

ISBN: 0-9624191-8-4

No patent liability is assumed with respect to the use of the information contained herein. While every precau­ tioh has been taken in the preparation of this document, the author assumes no responsibility for errors or omissions. Neither is any liability assumed for damages resulting from the use ofthe Information contained herein.

The Williamson Amplifier

CONTENTS

Page

Introduction

6

Basic Requirements:

7

Alternative Specifications (April 1947)

Details of Chosen Circuit and Its Performance

11

(May 1947)

NEW VERSION Design Data:

14

Modifications: Further Notes (August 1949)

Design of Tone Controls and Auxiliary Gramophone Circuits

20

(October and November 1949)

Design for a Radio Feeder Unit

30 (December 1949)

Replies to Queries Raised by Constructors

33

(January 1950)

Modificationsfor High-impedance Pickups and Long-playing Records 34 (May 1952)

5

Introduction

Introduced by Wireless World in 1947 as merely one of a series of amplifier designs, the "Williamson" has for several years been widely accepted as the standard of design and performance wherever amplifiers and sound reproduction are discussed.

Descriptions of it have been

published in all the principal countries of the world, and so there are reasonable grounds for assuming that its widespread reputation is based solely on its qualities. This booklet includes all the articles written by D. T. N. Williamson on the amplifier.

Both the 1947 and 1949 versions are reprinted, as the

alternative output transformer ratios cover a wide range of require­ ments.

Modifications and additions include pre-amplifier circuits and

an r.f. unit, with recently published information on adaptation to high­ impedance pickups and correction for 33t r.p.m. records. We would stress the importance, if the full potentialities of the amplifier are to be realized, of following the author's recommendations in detail.

Even in the U.S.A., where several modified versions have

been described, many users adhere to the designer's exact specification with the original valve types.

It is not the circuit alone, but the

properties of the valves and such components as the output transformer, together with the welding of theory and practice into a rational layout, which produce the results. Editor, Wireless World.

6

The Williamson Amplifier

Basic Design Requirements:

A lternative Specifications

R E CE N T improvements in the field of commercial sound recording have made prac­ ticable the reproduction of a wider range of frequencies than hitherto. The useful range of shellac pressings has been ex­ tended from the limited 50-8,000 cis which, with certain notable exceptions, has been standard from 1930 until the present, to a range of some 20-15,000 cis. This increase in the frequency range has been accompanied by an overall reduction in distortion and the absence of peaks, and by the recording of a larger volume range, which combine to make possible a standard of reproduction not pre­ viously attainable from disc re­ cordings. Further improvements, notably the substitution of low­ noise plastic material for the present shellac composition, are likely to provide still further enhanced performance. The resumption of the television service with its first-class sound quality. and the possible extension of u.h.f. high-quality trans­ missions, increase the available sources of high-quality sound. Full utilization of these record­ ings and transmissions demands reproducing equipment with a standard of performance higher than that which has served in the past. Extension of the frequency range, involving the presence of large-amplitude low-frequency sig­ nals, gives greater likelihood of intermodulation distortion in the reproducing system, whilst the enhanced treble response makes this type of distortion more readily detectable and undesirable. Reproduction of sound by elec­ trical means involves the ampli­ fication of an electrical waveform which should be an exact counter­ part of the air pressure waveform which constitutes the sound. The purpose of the amplifier is to produce an exact replica of the electrical input voltage waveform at a power level suitable for the

operation of the loudspeaker. This in turn reconverts the elec­ trical waveform into a corres­ ponding sound pressure waveform, which in an ideal system would be a replica of the original. The performance of an amplifier intended to reproduce a given waveform is usually stated in terms of its ability to reproduce accurately the frequency com­ ponents of a mythical Fourier analysis of the waveform. While this method is convenient and indeed corresponds to the manner in which the mechanism of the ear analyses sound pressure wave­ forms into component frequencies and thereby transmits intelligence to the brain, the fact that the function of the system is to repro­ duce a waveform and not a band of frequencies should not be neglected. Sounds of a transient nature having identical frequency contents may yet be very different in character, the discrepancy being in the phase relationship of the component frequencies. The requirements of such an amplifier may be listed as : ­ (I) Negligible non-linear dis­ tortion up to the maximum rated output. (The term .. non-linear distortion" includes the produc­ tion of undesired harmonic fre­ quencies and the intermodulation of component frequencies of the sound wave.) This requires that the dynamic output/input char­ acteristic be linear within close limits up to maximum output at all frequencies within the audible range. (2) (a) Linear frequency re­ sponse within the audible fre­ quency spectrum of 10-20,000 cis. (b) Constant power handling capacity for negligible non-linear distortion at any frequency within the audible frequency spectrum. This requirement is less strin­ gent at the high-frequency end of the spectrum, but should the maximum power output/frequency response at either end of the

spectrum (but especially, at the low-frequency end) be substan­ tially less than that at medium frequencies, filters must be arranged to reduce the level of these frequencies before they reach the amplifier as otherwise severe intermodulation will occur. This is especially noticeable during the reproduction of an organ on incorrectly designed equipment where pedal notes of the order of 16-20 c / s cause bad distortion, even though they may be in­ audible in the sound output. (3) Negligible phase shift with­ in the audible range. Although the phase relationship between the component frequencies of a complex steady-state sound does not appear to affect the audible quality of the sound, the same is not true of sounds of a transient nature, the quality of which may be profoundly altered by disturb­ ance of the phase relationship between component frequencies. (4) Good transient response. In addition to low phase and fre­ quency distortion, other factors which are essential for the accu­ rate reproduction of transient wave-forms are the elimination of changes in effective gain due to current and voltage cut-off in any stages, the utmost care in the design of iron-cored components, and the reduction of the number of such components to a minimum. Changes in effective gain during " low-frequency" transients occur in amplifiers with output stages of the self-biased Class AB type, causing serious distortion which is not revealed by steady-state measurements. The transient causes the current in the output stage to rise, and this is followed, at a rate determined by the time constant of the biasing network, by a rise in bias voltage which alters the effective gain of the amplifier. (5) Low output resistance. This requirement is concerned with the attainment of good 7

The Williamson Amplifier

The salient fea­ tures of these methods are of interest. Push-pull triode valves without INPUT the refinement of negative feed­ back form the mainstay of pre­ sent-day high­ (~) (b) equip­ fidelity ment. A stage of Fig. I. Output/input characteristics (a) without this type has a feedback (b) with negative feedback. number of dis­ advantages. With reasonable frequency and transient response efficiency in the power stage from the loudspeaker system by such an arrangement cannot be ensuring that it has adequate made to introduce non-linearity to electrical damping. The cone an extent less than that represen­ movement of a moving-coil loud­ ted by about 2-3 per cent speaker is restricted by air loading, harmonic distortion. The output/ suspension stiffness and resistance, input characteristic of such a stage and electro-magnetic damping. In is a gradual curve as in Fig. the case of a baffle-loaded loud­ I (a). With this type of characteris­ speaker, the efficiency is rarely tic distortion will be introduced at higher than 5-10 per cent, and the all signal levels and intermodula­ air loading, which determines the tion of the component signal radiation, is not high. In order frequencies will occur at all levels. to avoid a high bass-resonance The intermodulation with such a frequency, the suspension stiffness characteristic is very considerable in a high-grade loudspeaker is and is responsible for the harsh­ kept low, and obviously the power ness and "mnshiness" which loss in such a suspension cannot characterizes amplifiers of this be large. Electro-magnetic damp­ type. In addition, further non­ ing is therefore important in linearity and considerable inter­ controlling the motion of the cone. modulation will be introduced by This effect is proportional to the the output transformer core. current which can be generated If the load impedance is chosen in the coil circuit, and is therefore to give maximum output the proportional to the total resistance load impedance/output resistance of the circuit. Maximum damp­ ratio of the amplifier will be about ing will be achieved when the coil 2, which is insufficient for good is effectively short-circuited, hence loudspeaker damping. the output resistance of the It is difficult to produce an amplifier should be much lower adequate frequency response char­ than the coil impedance. acteristic in a multi-stage ampli­ (6) Adequate power reserve. fier of this type as the effect of The realistic reproduction of orchestral music in an average multiple valve capacitances and the output transformer primary room requires peak power capa­ bilities of the order of 15-20 and leakage inductances becomes serious at the ends of the a.f, watts when the electro-acoustic transducer is a baffle-loaded spectrum. moving-coil loudspeaker system The application of negative feed­ back to push-pull triodes results of normal efficiency. The use in the more or less complete sol­ of horn-loaded loudspeakers may reduce the power requirement to ution of the disadvantages out­ the region of 10 watts. lined above. Feedback should be applied over the whole am­ The Output Stage plifier, from the output transform­ An output of the order of 15-20 er secondary to the initial stage as watts may be obtained in one of this method corrects distortion three ways, namely, push-pull introduced by the output trans­ triodes, push-pull triodes with former and makes no additional negative feedback, or push-pull demands upon the output capabili­ tet ro.les with negative feedback. ties of any stage of the amplifier. 8

The functions of negative feed­ back are:­ (a) To improve the linearity of the amplifier, and output transformer. (b) To improve the freqnency response of the amplifier and output transformer. (e) To reduce the phase shift in the amplifier and output trans­ former within the audible fre­ quency range. (d) To improve the low-fre­ quency characteristics of the out­ put transformer, particularly defects due to the non-linear relation between flux and magne­ tizing force. (e) To reduce the output resistance of the amplifier. (fl To reduce the effect of random changes of the para­ meters of the amplifier and supply voltage changes, and of any spurious defects. A stage of this type is capable of fulfilling the highest fidelity requirements in a sound repro­ ducing system. The output/input characteristic is of the type shown in Fig. I (b), and is virtually straight up to maximum output, when it curves sharply with the onset of grid current in the out­ put stage. Non-linear distortion can be reduced to a degree repre­ sented by less than o. I per cent harmonic distortion, with no audible intermodulation. The frequency response of the whole amplifier from input to output transformer secondary can be made linear, and the power handling capacity constant over a range considerably wider than that required for sound reproduc­ tion. The output resistance, upon which the loudspeaker usual!" depends for most of the damping required, can be reduced to a small fraction of the speech coil impedance. A ratio of load im­ pedance/output resistance (some­ times known as "damping fac­ tor ") of 20-30 is easily obtained, " Kinkless " or "beam" ou t­ put tetrodes used with negative feedback can, with care, be made to give a performance midwav between that of triodes with and without feedback. The advantages to be gained from the use of tetrodes are increased power effi­ ciency and lower drive voltage requirements.

The Williamson Amplifier

It must be emphasized that the characteristics of the stage are dependent solely upon the char­ acter and amount of the negative feedback used. The feedback must remain effective at all frequencies within the a.f. spectrum under all operating con­ ditions, if the quality is not to degenerate to the level usually associated with tetrodes without feedback. Great care must be taken with the design and opera­ tion of the amplifier to achieve this, and troubles such as parasitic oscillation and instabilitv are liable to be encountered. ­ When equipment has to be operated from low-voltage power supplies a tetrode stage with negative feedback is the only choice, but where power supplies are not restricted, triodes are preferable because of ease of operation and certainty of results. It appears then that the design of an amplifier for sound repro­ duction to give the highest possible fidelity should centre round a push-pull triode output stage and should incorporate negative feed­ back. The most suitable types of valve for this service are the PX;Zs and the KT66. Of these the Kt66 is to be preferred since it is a more modern indirectly-heated type with a 6.3-volt heater, and will simplify the heater supply problem. Triode-connected it has characteristics almost identical with those of the PX2S. Using a supply voltage of some 440 volts a power output of IS watts per pair may be expected.

The Output Transformer The output transformer is prob­ ably the most critical component in a high-fidelity amplifier. An incorrectly designed component is capable of producing distortion which is often mistakenly attribu­ ted to the electronic part of the amplifier. Distortion producible directly or indirectly by the output transformer may be listed as follows : ­ (a) Frequency distortion d lie to low winding inductance, high leakage reactance and resonance phenomena. (b) Distortion due to the phase shift produced when negative feedback is applied across the transformer. This usually takes

the form of parasitic oscillation due to phase shift produced in the high frequency region by a high leakage reactance. (c) Intermodulation and har­ monic distortion in the output stage caused by overloading at low frequencies when the primary inductance is insufficient. This is primarily due to a reduction in the effective load impedance below the safe limit, resulting in a very reactive load at low frequencies. This may cause the valves to be driven beyond cut-off since the load ellipse will tend to become circular. (d) Harmonic and intermodula­ tion distortion produced by the non-linear relation between flux and magnetizing force in the core material. This distortion is always present but will be greatly aggra­ vated if the flux density in the core exceeds the safe limit. (e) Harmonic distortion intro­ duced by excessive resistance in the primary winding. The design of a practical trans­ former has to be a compromise between these conflicting require­ ments. . At a low frequency fb' such that the reactance of the output trans­ former primary is equal to the resistance formed by the load resistance and valve a.c, resist­ ances in parallel, the output voltage will be jdb below that at medium frequencies. At a fre­ quency 3fb the response will be well maintained, the transformer reactance producing only 20 0 p h a s e angle. Similarly at the high frequency end of the spectrum the response will be jdb down at a frequency ft such that the leakage reactance is equal to the sum of the load and valve a.c. resistances. Again at a frequency ftl3 the

w u

z

< .... u

:::>

c :!:

'A ALTERNATING EXCITATION VOLTAGE

Fig. 2. Variation of iron-cored inductance with a.c, excitation,

response will be well maintained. If then the required frequency range in the amplifier is from 10-20,000 cis, fb may be taken as 3.3 cis and ft as 60 kc/s. A trans­ former which is only 3db down at frequencies as widely spaced as these would be difficult to design for some conditions of operation, and where this is so the upper limit may be reduced, as the energy content of sound at these frequencies is not usually hig-h. The limiting factor will be the necessity of achieving stability when feedback is applied across the transformer, i.e., that the loop gain should be less than unity at frequencies where the phase shift reaches 180°. To illustrate the procedure, consider the specification of an output transformer coupling two push-pull I{T66 type valves to a Is-ohm loudspeaker load. Primary load impedance= ro.ooot)

. = T urns ratio

JIO'.._ OOO- =

25. 8 :1

15

Effective a.c. resistance of valves = 2500 f! Low-frequency Response Parallel load and valve resist­ ance = 25°o._~~,000 = 2000!.l 12,5 00 fb = 3·3 c!S(w b = 2 I ) response should be jdb down. Primary incremental inductance 2000 L = - - = 95 H. 21

High-frequency Response Sum of load and a.c. resistances = 10,000 + 2S00 =

12SOO

f!

At ft = 60 kc/s (Wt = 37 6,000) response should be jdb down. 12,5 00 Leakage reactance 37 6 = 33 m H. A zo-wat t transformer having 10 primary and 8 secondary sections and using one of the better grades of core material can be made to comply with these requirements. Winding data will be giv!"n in an appendix (see page II). Some confusion mav arise when specifying an output - transformer as the apparent inductance of the windings will vary greatly with the method of measurement. The inductance of an iron-coreI 9

The Williamson Amplifier

(a)

(b)

(c)

Fig. 3.

Block diagrams of circuit arrangements discussed in the text.

component is a function of the excitation, the variation being of the form shown in Fig. 2. The exact shape of the curve is dependent on the magnetization characteristic for the core rna terial. The maximum inductance, corresponding to point C occurs when the core material is nearing saturation and is commonly 4-6 times the "low excitation" or " incremental" value at A, which corresponds to operation near the origin of the magnetization curve. In a correctly designed output transformer the primary induct­ ance corresponding to the voltage swing at maximum output at 50 c / s will lie in the region of B in Fig. 2. In specifying the component, the important value is the incre­ mental inductance corresponding to point A, since this value deter­ mines the frequency response at low outputs.

Phase Shift The red uction of phase shift in amplifiers which are to operate with negative feedback is of prime importance, as instability 10

will result, should a phase shift of 180 0 occur at a frequency where the vector gain of the amplifier and feedback network is greater than unity. The introduction of more than one transformer into the feedback path is likely to give rise to trouble from insta­ bility. As it is desirable to apply feedback over the output trans­ former the rest of the amplifier should be R-C coupled.

to a low value as it contains the minimum number of stages. The arrangement, however, has a number of disadvantages which render it unsuitable. The input voltage required by the phase splitter is rather more than can be obtained from the first stage for a reasonable distortion with the available h.t, voltage, and in addition the phase splitter is operating at an unduly high level. The gain of the circuit is low even if a pentode is used in the first stage, and where a low-impedance loudspeaker system is used, in­ sufficient feedback voltage will be available. The addition of a push-pull driver stage to the previous arrangement, as in Fig. 3 (b), provides a solution to most of the difficulties. Each stage then works well within its capabilities. The increased phase shift due to the extra stage has not been found unduly troublesome provided that suitable precautions are taken. The functions of phase splitter and push-pull driver stage may be combined in a self-balancing " paraphase" circuit giving the arrangement of Fig. 3 (c). The grid of one drive valve is fed directly from the first stage, the other being fed from a resistance network between the anodes of the driver valves as shown in Fig. 4. This arrangement forms a good alternative to the preceding one where it is desirable to use the minimum number of valves.

Alternative Circuits Although the amplifier may contain push-pull stages it is desirable that the input and output should be "single ended" and have a common earth terminal. Three circuit arrangements suggest themselves. The block diagram of Fig. 3 (a) shows the simplest circuit arrange­ ment. The output valves are preceded by a phase splitter which is driven by the first stage. The feedback is taken from the output transformer secondary to the cathode of the first stage. This arrangement is advantageous in that the phase shift in the amplifier can easily be reduced

Fig. 4. .. Paraphase" circuit combining the functions of phase splitter and push-pull driver stages.

The Williamson Amplifier

Details of Chosen Circuit and Its Performance T

H E considerations under­ lying the design of a high­ quality amplifier were dis­ cussed in the first part of this article. A circuit of the complete amplifier is shown in Fig. 5. This follows the basic arrangement of Fig. 3(b). The design of the indi­ vidual stages will not be treated in detail. but a review of the salient features may be of value. As a measure of standardization all valves except those of the out­ put stage are type L63. triodes of about 8.000 ohms a.c. resistance. Initial Stages.-In order to

keep the phase shift in the ampli­ fier at low frequencies as small as possible the first stage has been directly coupled to the phase splitter, eliminating one R-C coupling. The first two stages are thus designed as a single entity. The phase-splitter section, which consists of a triode with equal loads in anode and cathode cir­ cuits, operates partly as a cathode follower, its grid being some 100 V positive with respect to chassis. The anode of the first triode is also arranged to be about 100 V posi­ tive and is coupled to the phaseCH I

4'4mA

!s'ZSmA

splitter grid. Due to the cathode­ follower action of V. the operating conditions are not critical and no trouble is likely to be encountered from normal changes in valve parameters. The cathode bias resistor of V l ' to which feedback is applied from the output trans­ former secondary, is kept as small as practicable to avoid gain reduc­ tion in the first stage, due to series feedback. Driver Stage.-The output from the phase-splitter is taken to the push-puU driver stage. Provision is made for varying the load reCHz

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R7

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l-

Rs

liT

..!:ll

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R4

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Fig. 5. Circuit diagram of complete amplifier. Voltages underlined are peak signal voltages at IS watts output. CIRCUIT VALUES 1 MO 1 watt ± 20 per cent

33,000 0 1 watt ± 20 47,000 0 1 watt ± 20 470 0 1 watt ± 10 R~ R 6 , R 8 , R 7 22,000 01 watt ± 10 R s, R g 0.47 MO 1 watt ± 20 390 0 1 watt ± 10 RIO R n , R 18 39,000 0 2 watt ± 10 25,000 0 1 watt wireR 12 wound variable. R U,R 19 0.1 MO 1 watt ± 20 R1 R2 R8

.. .

..

... .

.

1,000 0 1 watt ± 20 per cent 100 0 1 watt ± 20 100 0 2 watt wirewound variable. 160 0 3 'Watt ± 20 R 22 R 23 , R 2 4. 100 O! watt ± 20 1,2oov'speech coil impedance, R 26 1 watt. C1 , C2 , C6 8 /LF 450 V, wkg. c; C4. 0.05 /LF 350 V, wkg. 0.25/LF 350 V, wkg. C8 , C7

R 16,R20 R 18 , R 1S R)7,R 2 1

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Ca Cg CHI CH 2

8 /LF 550 V, wkg. 8 I£F 600 V, wkg.

;

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,

30 H at 20 mA (min.)

10 H at 150 mA (min.) Power transformer. T Secondary 425-0-425 V. 150 mA (min.) 5 V. 3A,6.3 V. 4A, c.t. 1VI to V.. L63 KT66 V6 , V8 U52. V7 11

N Y

I'

The Williamson Amplifier

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....oz

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WITHOUT FEEOBACK

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Fig. 6. Input-output characteristic and harmonic distortion curves, with and without feedback. (Right)­ Oscillograms of input-output characteristic ; left-hand column, without feedback; right-hand column, with feedback. (I) At 300 cIs with slight overload (2) At 300 cis, output voltage 15% below maximum. (3) and (4) Conditions as in (I) and (2) respectively, but at 30 cis.

sistors of this stage which, in con­ junction with a common unby­ passed cathode bias resistor, allows a considerable range of adjustment to be made in the drive voltages to the output valves to compensate for any inequality in gain. Output Stage.-The balance of quiescent anode current in the output stage is a matter of some importance, as it affects the per­ formance of the output trans­ former to a marked degree. In this amplifier, provision is made, by means of a network in the cathode circuits of the KT66 valves, for altering the grid bias of each valve, giving complete control of the static conditions of the stage. A feature of this arrangement is that the valves operate with a common unby­ passed cathode bias resistor, which assists in preserving the balance of the stage under dynamic conditions. Output Transformer. - The turns ratio of the output trans­ former will be determined by the impedance of the loudspeaker load. It is convenient to make each secondary section of such an 12

impedance that by series-parallel arrangement a number of suitable load impedances may be provided utilizing all the sections of the transformer. A suitable value of impedance is 1.7 ohms per sec­ tion, giving alternatives of I. 7, 6.8, 15.3, 27 ohms, etc. Winding data for a suitable transformer are given in the Appendix. Negative Feedback Network.­ The design of this amplifier is such that no difficulty should be experi­ enced in the application of nega­ tive feedback up to a maximum of some 30 db. Provided that the threshold of instability is not reached, the benefits of negative feedback increase as the amount of feedback is increased, at the sale expense of loss of gain, but there will be little if any audible improvement to be gained with this amplifier by increasing the amount of feedback beyond 20 db. The feedback network is a purely resistive potential divider, the bottom limb of which is the cathode bias resistor of the first stage. With component values as

specified no trouble should be ex­ perienced from instability due to the effects of unintentional posi­ tive feedback. Should instability arise it will probably appear as oscillation at a supersonic fre­ quency. This may be transient, occurring only at some part of the cycle when the amplifier is oper­ ated near maximum output. Its cause may be bad layout or an output transformer with a higher leakage reactance than specified, or it may be due to resonance in the output transformer. A remedy, which should only be used as a temporary measure, is to reduce the high-frequency response of one of the amplifier stages, so reducing the loop gain at the frequency of oscillation to a value below unity. This may conveniently be done by connect­ ing a small capacitor (say 200 pF) in :series with a 5,000 n resistor from the anode of V J to chassis.

Performance Linearity.-The linearity of the amplifier is well illustrated by the series of oscillograms. These show that, up to maximum output, the linearity is of a high order, and

The Williamson Amplifier

that the overload characteristic is of the desirable type shown in Fig. I(b) in the previous issue. . The improvement due to the application of negative feedback, especially at low frequencies, is clearly demonstrated by the oscillograms. Equipment for measuring inter­ modulation products was not available, but measurement of the total harmonic distortion was made with an input frequency of 400 c / s. The result is shown in Fig. 6, from which it will be seen that the harmonic distortion at maximum rated output (15 watts) is less than o. I per cent. Inter­ modulation, with this degree of linearity, is not present to an audible degree. Frequency Response.-The fre­ quency response of the amplifier is greatly dependent upon the characteristics of the output trans­ former. In the amplifier tested, the output transformer had a resonance at about 60 kc / s which caused a sharp dip of 2.6 db around this frequency. The char­ acteristic within the audible range from 1o-20,oooc/s is linear with­ in 0.2 db. Phase Shift.-The excellence of the frequency response character­

istic indicates that little phase shift is present. Phase shift is only apparent at the extremes of the a.f. spectrum and never exceeds a few degrees. Output Resistance.-The out­ put resistance of the amplifier is 0.5 ohms measured at the ry-ohm output terminals. Noise Level.-In the amplifier tested, the measured noise level was 85 db below maximum output. The noise in this amplifier was, however, almost entirely 50 c / s hum, caused by coupling between the mains and output trans­ formers. By more careful ar­ rangement of these components it appeared that the noise level could be reduced to better than 100 db below maximum output. If desired, the power output of the amplifier may be increased beyond 15 watts by the use of several pairs of output valves in parallel push-pull. The output transformer, power supply and bias arrangements, and the feed­ back resistor R 2 5 will require to be modified. Amplifiers of this design with power outputs up to 70 watts have been produced. Listening tests carried out in conjunction with a wide-range loudspeaker system have fully

+I db

.....,

01.-­ I

8

8.

o

8. o

~

FREQUENCY IN CYCLES PER SECOND

Fig. 7. Frequency response (without feedback) of 20 watt output trans­ former described in appendix. Generator resistance 2,500n load resistance I.7n. Measured with 5V r.m.s, on primary. At higher excitations the bass response improves progressively up to saturation.

supported the measured perform­ ance. No distortion can be de­ tected, even when the amplifier is reprod ucing organ music includ­ ing pedal notes of the 20 c / s order, which reach the threshold of maximum output. Transients are reproduced with extreme fidelity; tests using a direct microphone circuit with noises such as jingling keys reveal extraordinary realism. The amplifier can be described as virtually perfect for sound­ reproducing channels of the high­ est fidelity. It provides an ideal amplifier for sound-recording pur­ poses, where " distortion less " amplification and low noise le-ve-l are of prime importance. APPENDIX. Output Transformer. Specification. Primary load impedance = 10,000 ohms c.t , Secondary load impedance = 1.7 ohms per sec­ tion. Turns ratio = 76 : t , Primary inductance e- 100 H (min.) Leakage inductance= 30 mB (max Windin~

Data. Core: din stack of Pattern No. 2SA "Super Sileor" lamiua tions (Magnetic and Electrical Alloys, Burnbank, Hamilton, Lanarks.) The winding consists of two identical interleaved coils, each l~ill wide, wound on [lin x I tin paxolin formers. On each former is wound: 5 primary sections each consisting of 5 laye-rs (SS turns per layer) of 30 s.w.g. enamelled copper wire interleaved with 2 mil. pappr, alter­ nating with 4 secondary sections. each consisting of 2 layers (2.~. The author takc."i the reader deep into his design corwidcranons. offcrin~ practical advice on how [0 build the units pim; concise instructions on setting up tbc new amp. A cult classic.

Authored by the fOlIn(lcrofSttTeopltiJe ma~azine, rhis new ben seller is it comprehensive overview of over 1,900 technical and subjective audio terms explained in precise yet at times humorous fashion. Three edition': Softcover (S); Hardcover with Dust [acker (H); and Limited, Auto­ graphed Hardcover with Gold-Embossed l\inding FAll - - ­

~!S

*C ss

RSlf R47

CS4

R48

C23

Rn

R

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I~O

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1,000

1 1,500 1,600

FREQUENCY (kc/.)

Fig.

21.

Curve relating tuned circuit parameters and resonance frequency.

70

80

90

100

110

120

IlO

140

NUMBER OF TURNS

Fig, 22. Curve relating inductance and num­ ber of turns for windings discussed in ten.

31

The Williamson Amplifier

COIL FORMERS

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